"Does 'HAM' stand for 'Haven't Any Money'"?

Many amateur radio operators (myself included) are cheapskates.  After all, this is a hobby, and rather than spending money on things like coax, antennas, clothes and food, we'd rather spend our money on the new rig, or on that 'neato' junque that we just saw at the swapmeet (or on Ebay...)  I'm not speaking for all of us, mind you, but I'm sure you know someone that fits the description.

In the same spirit, we'll often try to make do with something that we have on hand when, in our heart, we really know that it won't work to well.  We are often willing to spend a lot of time trying to make it work when, were we were paying ourselves minimum wage, we could have bought the real thing.

On ATV, the question is often asked:  "Can't I use my existing rooftop TV antenna for 70cm ATV and get a good picture?"  The intent here is save money by trying using the antenna/coax that you probably already have connected to your TV.

The answer is:  "Yes, you can't."

Well, probably.

Since I was curious about this too, so I did some measurements using the following antennas.  A "nickname" of each antenna is in quotes just after each letter to aid in deciphering the charts below:

A - "RSANT"  Radio Shack VHF/UHF combination, Model VU-75XR.  "18" elements (according to the catalog)  which translates to 3 VHF "dipoles" (each "dipole" is counted as 2 elements since it is a log-periodic antenna) and on UHF it has 4 directors, 1 driven element, and 6 "reflector" elements on the "vee" portion.  The 300-75 ohm balun was mounted on the antenna.  The manufacturer of this antenna states a gain of 6.4dbi on UHF.
B - "UVANT"  A large VHF/UHF combination, unknown manufacture (about 15 years old.)  11 VHF "dipoles", on UHF it has 6 directors, 1 driven element, and 6 "reflector" elements on the "vee" portion.  The 300-75 ohm balun was mounted on the antenna.
C - "TVYAGI"  10 element yagi cut for to channel 14, made by Blonder-Tongue.
D - "HTANT"  2meter/440 HT dual-bander rubber duck antenna.  (1/4 wave on 2 meters, 5/8 wave on 70cm, made by Premier.)  The antenna was held horizontally and rotated until a consistent peak was found.  (The antenna was broadside to the transmitter for the peak.)
E -  "8LYAGI"  8 element 70cm yagi, homebrew broadband.  Antenna gain minus feedline loss is approximately 10dbi.
F -  "RABBIT"  Rabbit Ear antenna with 18" length of twinlead and balun.  Various combinations of antenna rotation, minimal, partial, and full telescoping of the elements was tried.  Interestingly, the signals were within 2 db of the best value no matter how the elements were telescoped or collapsed when the antenna was oriented for best signal (i.e. broadside to the transmitter site.)

Sadelco MiniMax 800 TV signal strength meter

• All measurements were made with a Sadelco MiniMax 800 signal strength meter.  This meter is rated for an accuracy of +- 0.25 db over a 5-862 MHz operating frequency range and lab measurements tend to corroborate this.
• All measurements were all made on the same site and with a clear view of the transmitter site (which was 15 miles distant and over 4700 feet higher in elevation.)
• For antennas A, B, and C a short (18") length of RG-6 was used to connect the antenna to the signal meter.
• Antenna D was connected directly to the signal meter via an F to BNC adapter.
• For antenna E, approx. 15 feet of 9913-type coaxial cable was used to connect the antenna to the signal meter.
• The balun on antenna F was connected directly to the signal meter.
• Even though antennas D and E are ostensibly 50 ohms, no attempt was made to compensate.  It should be noted that the loss of a balun to convert 75 ohms to 50 ohms would likely incur as much loss as the mismatch itself!  (The mismatch between 50 and 75 ohms only represents a VSWR of 1.5:1 anyway...)
• The signal level readings represent those of the visual carrier only.

• The ATV signal source was the WB7FID repeater on 426.25 MHz.
• The Channel 14 and Channel 30 signal sources were from KJZZ-TV and KUWB respectively.
• All three signal sources originate from the top of the Oquirrh mountains, about 15 miles west of the receive site.  These sites are within 3 degrees of each other in terms of bearing.
• All signals are horizontally polarized.

• These results are in dbmV.  This is "db with respect to 1 millivolt of RF at 75 ohms."  This may be an unfamiliar number to many, but we are more interested in performance differences between antennas than the absolute signal strengths.  To provide a more familiar reference, signal strengths in microvolts are given in parenthesis, along with the actual observed picture quality.
• For a discussion of the "P" ratings and about how much signal it takes to get a particular quality signal, go the the "P's and Q's of Video Signals" page.

• A - RSANT:  -23 dbmV (approx. 71 microvolts, a P2 ["noisy"] picture)
• B - UVANT:  -15 dbmV (approx. 178 microvolts, a P3 picture)
• C - TVYAGI: -20 dbmV (approx. 100 microvolts, a "noisy" P3 picture)
• D - HTANT:  -19 dbmV (approx. 112 microvolts, a "noisy" P3 picture)
• E - 8LYAGI: -10 dbmV (approx. 316 microvolts, a P4 picture)
• F - RABBIT: -17 dbmV (approx. 141 microvolts, a P3 picture)

• A - RSANT:  +15 dbmV (approx. 5.6 millivolts)
• B - UVANT:  +25 dbmV (approx. 17.8 millivolts)
• C - TVYAGI: +27 dbmV (approx. 22.4 millivolts)
• D - HTANT:  (No reading taken)
• E - 8LYAGI: (No reading taken)
• F - RABBIT: +11 dbmV (approx. 3.5 millivolts)

• A - RSANT:  +18 dbmV (approx. 7.9 millivolts)
• B - UVANT:  +19 dbmV (approx. 8.9 millivolts)
• C - TVYAGI: -2 dbmV (approx. 794 microvolts)
• D - HTANT:  (No reading taken)
• E - 8LYAGI: (No reading taken)
• F - RABBIT: (No reading taken)

"If you use a VHF/UHF TV antenna for watching ATV, don't expect great results."

Clearly, the amateur-band antennas worked better than the non-ham antennas but I didn't expect the VHF/UHF TV-type antennas to work quite as badly as they did.  As you can see, you would be better off with either a rubber duck or a pair of rabbit-ear antennas (on your roof!)  than antenna A (the Radio Shack VU-75XR.)  Antenna B (the large 15 year old VHF/UHF antenna) performed quite a bit better for ATV than did A, but even it doesn't even perform as well as a 3 element yagi might!

It is interesting to compare the performance of antenna C (the 10 element yagi for channel 14) against the other antennas.  It can be reasonably assumed that this antenna is working about as well as a standard 10 element yagi might (which would imply a gain of about 14 dbi.  By extrapolation, antenna A would have a gain of 2dbi and antenna B would have a gain of 12dbi.

By comparison, if we assume that antenna E (the 8 element 70cm yagi) has a gain of 10 dbi (this figure includes feedline loss) then we see that, at 426.25 MHz, A has a gain of 1 dbi (1db worse than a dipole!) and antenna B has a gain of about 9dbi - about the same gain as a 3 element yagi.

While these numbers may seem pretty poor, these readings jibe with experience:  Most VHF/UHF TV antennas tend to drop in gain as frequency goes down.  This would make sense because, to get better "low end" gain, the elements (and the antenna itself) need to be longer - and this would make it more expensive.  Looking at the signal strengths of channel 30 on antennas A and B reveals that these two antennas perform more or less identically.  Visual comparison of the two antennas shows that they are similarly constructed - but with one critical difference:  The UHF elements on the B antenna are slightly longer and are constructed in a way that makes them intrinsically more broadbanded, hence the poorer performance of A at the lowest frequency (e.g. Channel 14.)

"How much power would the ATV repeater have to run to make a typical VHF/UHF TV antenna work well?"

At present, antenna A yields a signal of 71 microvolts at the antenna.  Measured at the end of about 60 feet of RG-6 coaxial cable and going through one splitter, we end up with a signal strength of -29 dbmV (about 35 microvolts.)  This results in a noisy (P2) picture.

Presently, the WB7FID ATV repeater runs about 60 watts or so output into the filter/combiner/isolator network.  If we wanted to improve the P2 that we are currently receiving to, say, a "good P3" (a perfectly watchable picture, but with some noise) we would have to increase the signal strength from the current -29 dbmV (35 microvolts) to about -15 dbmV (approximately 178 microvolts) - a difference of 14db.  This means that the transmit power would have to be raised from 60 watts to 1500 watts - that's full legal power!  Conversely, if we were to move closer to the repeater's transmit antenna, we'd have to be closer than one-forth the distance we started with in order to get that 14 db improvement!  (In our case, we'd have to move from 15 miles to 3 miles which would put us on the slopes of the mountains...)

As you might suspect, we have neither the budget or the physical space required to accommodate such a power level.

On the other hand, if someone were to put up a relatively small 70cm yagi (say, a 5 element yagi - which would be about 2 feet long) you'd end up with a gain of about 11 dbi or so.  At the antenna you'd get a signal strength of approximately -11 dbmV (about 280 microvolts) at the location above.  With this much signal from the antenna, we could lose 4 db in our coax (nearly 100 feet of good quality RG-6 coax) and still have as much signal as we'd get if we were to "up" the repeater's power to 1500 watts and using our rooftop VHF/UHF antenna.

Concluding conclusions:

If you do want to watch ATV, you probably won't get a decent signal from your rooftop antenna unless you live very close to the repeater and you do not have excessive feedline loss caused by splitters.  If you have a TV signal amplifier (especially an inline antenna-mounted type) then you may get some pretty reasonable signals if you are in the Salt Lake valley.  Don't forget that if you have line-of-sight to Farnsworth Peak, even a fairly modest (10 element) 70cm yagi will work pretty well anywhere in the Provo to Ogden area.

While the WB7FID repeater will be improved as time goes on, chances are that we will not be increasing the power so much that there will be a "night and day" difference.  The best "bang for the buck" can be had by a small-to-modest investment, by the person who wishes to look at the ATV repeater, in antenna and feedline.  From Ogden to Provo the signals are strong enough that a relatively small beam (a homebrew antenna, for example) fed with a reasonably short run of coax will result in very good pictures - and even these can be improved upon with only a bit more effort!

For further information about signal strength in various areas along the Wasatch Front, go to the Receiving the WB7FID ATV Repeater At Your QTH page.

Go to the Utah ATV Home Page...